Bronchial epithelium model

Figure 3a shows the simulation model of bronchial epithelium. Six types of cells, as shown in Fig. 3b, are distributed in the tissue segment. Tissue geometry modelling in this study was in line with that in previous studies^20,52; the experimental data measured by Mercer et al.²² and Mercer et al.⁵⁴ were used. A small segment (400 µm × 400 µm in area and 57.8 µm in thickness) of bronchial epithelium was filled with cell nuclei. Additional 6 µm-thick cilia (serous) and 5 µm-thick mucous layers were attached to the luminal surface⁵⁵. Liquid water composition was used for the epithelium section at a density of 1.0 g/cm³ for cell nuclei and the serous and mucous layers and at a density of 1.05 g/cm³ for the remaining volume⁵⁶. This segment represents the epithelium exposed to radon progeny in the airway carinal ridge. The nuclei of six cell types, including pre-ciliated, ciliated, goblet, basal, indetermined, and secretory cells, were considered as target bodies. The 5 μm-thick mucous layer was the source reservoir. Radon progeny were assumed to be uniformly mixed within the mucous layer, in accordance with the ICRU Report 88²³.

Dimensions of (a) the bronchial epithelium section for the simulation of alpha-particle emissions from radon progeny within the mucous layer and (b) six cell types constituting the epithelium section.

The 57.8 μm-thick tissue segment was divided into five regions, namely, four regions with 12 μm-thick layers and one region with 9.8 μm-thick layers from the luminal surface to the basement membrane. Each region was characterized by the frequencies of individual cell types, which were calculated by using the frequencies of lung cells per unit area of the basement membrane in the human large bronchi (Table ​(Table1)1) as reported by Mercer et al.²² and the relative portions in volume of individual cell nuclei in the region. Mercer et al.⁵⁴ provided the areal portions of six cell types in Table ​Table22 at six discrete depths of the bronchial epithelium. The areal portions were assumed to change linearly along the discrete depths. The continuous functions of areal portions were used to define the probability density function of each cell type in each region and to calculate the volume portions over the five regions. The nuclei of each cell type were located over the depth in each region according to the chance that was proportional to its areal portion. The continuity of the probability density function for each cell type was ensured in all regions by limiting the nuclear center within each region and allowing the nuclear volume to enter the adjacent regions. The nuclear centers did not overlap and did not reside on the boundary of the tissue segment.

Nucleus areal portions (%) for six cell types at varying depths from the surface of the bronchial epithelium⁵⁴.